The present invention relates to a cathode ray tube having a shadow mask, which is used for a television receiver, a computer display, and the like.
FIG. 3 is a cross-sectional view showing one example of a conventional color cathode ray tube. The color cathode ray tube 1 shown in FIG. 3 includes a substantially rectangular-shaped face panel 2 having a phosphor screen on its inner face, a funnel 3 connected to the rear side of the face panel 2, an electron gun 4 contained in a neck portion 3a of the funnel 3, a shadow mask 6 facing a phosphor screen 2a inside the face panel 2, and a mask frame 7 for fixing the shadow mask 6. Furthermore, in order to deflect and scan electron beams, a deflection yoke 5 is provided on the outer periphery of the funnel 3.
The shadow mask 6 plays a role of selecting colors with respect to three electron beams emitted from the electron gun 4. xe2x80x9cAxe2x80x9d shows a track of the electron beams. Hereinafter, this shadow mask 6 is referred to as a slot type shadow mask. The slot type shadow mask has a flat plate provided with a number of slot apertures by etching. The slot aperture is a substantially rectangular-shaped through aperture through which electron beams pass.
In a color cathode ray tube, due to the thermal expansion caused by absorbing electron beams, the electron beam through aperture is deformed. Consequently, a doming phenomenon occurs. That is, the electron beams passing through the electron beam through apertures fail to hit a predetermined phosphor correctly, thus causing unevenness in colors. Therefore, a tension force to absorb the thermal expansion due to the temperature increase of the shadow mask is applied in advance, and then the shadow mask is stretched and held to the mask frame. Thus, when stretching and holding the shadow mask as mentioned above, even if the temperature of the shadow mask is increased, it is possible to reduce the amount of displacement between a slot aperture of the shadow mask and phosphor strips of the phosphor screen.
FIG. 5 is a plan view showing a slot type shadow mask as an example of the shadow mask to which a tension force is applied mainly in the vertical direction of the screen. The direction illustrated by the arrow x is a horizontal direction of the screen, and the direction illustrated by the arrow y is a vertical direction of the screen. The slot apertures 8 are formed at constant pitches. Reference numeral 9 is referred to as a bridge, which is a portion between slot apertures 8. The bridge width has an effect on the mechanical strength of the shadow mask. More specifically, a narrower bridge width weakens the tension particularly in the horizontal direction. If the bridge width is increased in order to improve the mechanical strength, the aperture area of the slot aperture is reduced, thus deteriorating the luminance intensity.
Furthermore, the bridge width is related to the mechanical strength and luminance intensity, and the longitudinal pitch of the bridge also is related to the doming amount of the shadow mask. The shadow mask is stretched mainly in the longitudinal direction. This is because the thermal expansion in the longitudinal direction is absorbed by the tension force, while the thermal expansion in the lateral direction is transmitted in the lateral direction through the bridge.
FIG. 4 is a graph showing one example of the relationship between the bridge pitch and the doming amount (an example of a cathode ray tube for a 25-inch television is shown). FIG. 4 shows that the doming amount can be reduced as the bridge pitch is increased.
However, the conventional color cathode ray tube suffers from the following problem. When the longitudinal pitch of the bridge is increased, the doming amount can be reduced and the luminance intensity also is improved by the increase of the aperture area of the slot aperture. However, in this case, moire stripes easily occur, thus causing the deterioration of the image quality. The moire stripe means a mutual interference stripe between scanning lines (luminescent lines) of the electron beams arranged at constant intervals and the regular pattern of the electron beam through apertures of the shadow mask.
Furthermore, when the longitudinal pitch of the bridge is increased, the bridges themselves may appear as dots on the screen, or may be recognized as a pattern in which the bridges are piled up (a brick-like pattern).
On the contrary, when the bridge pitch is reduced, moire stripes are suppressed sufficiently and the bridges themselves are not noticeable. In this case, however, the shielded area of the scanning line is increased, and the luminance intensity is lowered, and the doming amount is also increased. In other words, it was difficult both to reduce the doming amount and to suppress the occurrence of moire stripes at the same time.
It is an object of the present invention to provide a cathode ray tube capable of reducing the doming amount and suppressing the occurrence of moire stripes at the same time by forming protruding portions facing each other inside the slot aperture in the tension style shadow mask.
According to the present invention, a cathode ray tube includes a mask frame formed in the form of a frame, and a shadow mask made of a flat plate containing a plurality of slot apertures, which is stretched and held onto the mask frame with a tension force applied in the longitudinal direction of the slot apertures, wherein the shadow mask has bridges linking the slot apertures arranged neighboring in the longitudinal direction, and the slot aperture has protruding portions facing each other and protruding from the both ends of the lateral direction of the slot aperture to the inside of the slot aperture. According to such a cathode ray tube, it is possible to reduce the doming amount due to the thermal expansion while suppressing the occurrence of moire stripes.
It is preferable in the above-mentioned cathode ray tube that the bridges and the protruding portions in the slot aperture line are displaced from the bridges and protruding portions in the neighboring slot aperture line, and the displacing amount is in the range from 1/2 to 1/5 with respect to the longitudinal pitch of the slot aperture. According to such a preferred cathode ray tube, it is possible to suppress the occurrence of moire stripes and also to suppress an occurrence of moire stripes in the oblique direction, which is caused by the displacement of bridges in the longitudinal direction, so that they are not observed significantly.
It is preferable that the protruding portions are arranged at the longitudinal pitch of 1 mm or less in the slot aperture, and the longitudinal pitch of the bridge is in the range from 1.5 to 30 mm. If the longitudinal pitch of the protruding portion is 1 mm or less, even in the case of a plural broadcasting method, it is possible to suppress an occurrence of moire stripes for each broadcasting by using one shadow mask structure. Furthermore, by setting the longitudinal pitch of the bridge in the range from 1.5 mm to 30 mm, it is possible to reduce the doming amount so as to be below a certain value, and to suppress the vibration of the shadow mask to fall within a practical level while securing a certain level of the luminous property and the mechanical strength.
Furthermore, it is preferable that the area of a pair of protruding portions arranged neighboring in the lateral direction of the slot aperture is in the range from 20% to 120% with respect to the area of one bridge. According to such a preferred cathode ray tube, it is possible to suppress the occurrence of moire stripes, while securing the luminous property.
Furthermore, it is preferable that the longitudinal pitch of the protruding portion differs in different parts of the shadow mask. With such a preferred configuration, it is possible to suppress an occurrence of moire stripes effectively in accordance with the part in the shadow mask where the moire stripes tend to occur.
Furthermore, it is preferable that the longitudinal pitch of the bridge differs in different parts of the shadow mask. With such a preferred configuration, the strength and the amount of heat transmission can be changed in accordance with the part in the shadow mask.
Furthermore, it is preferable that the width of the bridge in the longitudinal direction differs in different parts of the shadow mask. With such a preferred configuration, it is possible to change the strength of the bridge as needed, by taking into account the tension distribution of the shadow mask.
Furthermore, it is preferable that the width of the protruding portion in the longitudinal direction differs with the part in the shadow mask. With such a preferred configuration, the effect of shielding light beams by the protruding portions can be changed in accordance with the beam incident angle, to obtain the luminance intensity.
According to another aspect of the present invention, a cathode ray tube of the present invention includes a mask frame formed in the form of a frame, and a shadow mask made of a flat plate containing a plurality of slot apertures, which is stretched and held onto the mask frame with a tension force applied in the longitudinal direction of the slot apertures, wherein the shadow mask has bridges linking the slot apertures arranged neighboring in the longitudinal direction, the length of the slot aperture in the longitudinal direction is longer than the length in the central portion, and the slot aperture in the peripheral portion has protruding portions facing each other and protruding from the both ends of the lateral direction of the apertures to the inside of the slot aperture. Thus, in the central portion providing the reference tension force, the strength of the shadow mask can be secured while suppressing the thermal doming that tends to occur in the peripheral portion.